This invention relates generally to a wide frequency band capacitive microphone and method and more particularly to a microphone which includes a plurality of micromachined capacitive elements and more particularly to a microphone in which the capacitive elements are connected as a transmission line whereby vibrations of the microphone membranes modulate the length of the transmission line, and still more particularly to acoustic signal direction sensing microphones, pressure sensing transducers, wireless microphones and hydrophones.
A good microphone has several qualities: it must be able to detect a minimum pressure of 2xc3x9710xe2x88x925 Pa of pressure that corresponds to the minimum detectable sound level by the human ear, and it must operate over a broad frequency range. A recent stated frequency range requirement for many military applications is 1 mHz-200 KHz. Another requirement is robustness in that the microphone has to be operable outdoors without humidity and condensation deteriorating or destroying its operation. The microphone has to be small. It should withstand shock, high accelerations and high temperatures. Preferably, it should have directionality.
In order to have broad bandwidth, past designs have utilized damped structures where a backplate with many holes and a damping back pocket of air makes one side of a capacitor. A diaphragm, which makes the other side of the capacitor, is held with very soft springs over the back plate of the capacitor, and a traditional detection scheme is used to measure the displacement of the diaphragm. This approach suffers from low sensitivity, especially at low frequencies, and lack of robustness when it comes to use outdoors in humid environments.
It is an object of the present invention to provide a micromachined capacitive microphone operable over a wide frequency range.
It is another object of the present invention to provide a microphone which comprises a plurality of sealed micromachined capacitors.
It is a further object of the present invention to provide a self-cleaning micromachined microphone.
It is a further object of the present invention to provide an acoustic signal direction sensing capacitive micromachined ultrasonic microphone.
It is a further object of the present invention to provide an array of microphones operating from dc to 200 KHz frequency or more.
It is another object of the present invention to provide an array of microphones which can be co-located with respect to each other.
It is a further object of the present invention to provide a microphone for detecting sound and generating signals which can be transmitted to a remote receiver.
The foregoing and other objects of the invention are achieved by a wide-frequency band microphone including a plurality of sealed, evacuated, micromachined capacitive cells each including a membrane supported above a common conductive electrode. Each membrane supports a conductive electrode for movement therewith, whereby each membrane electrode forms a capacitor with the common conductive electrode. The capacitance of the capacitor varies with movement of the membrane responsive to received sound waves. Conductive lines interconnect said conductive electrodes to provide output signals. In another embodiment, the inductance of said lines and the capacitance of the capacitors forms a transmission line whose electrical length changes responsive to movement of said membranes. The membranes may be arranged in two independent parallel rows whereby the change in electrical length of the two transmission lines occur at times which are dependent upon the direction of the incoming sound, thereby permitting determination of the direction of the incoming sound source.